1. Field of the Invention
The present invention relates in general to cellular telephone systems and in particular to methods and systems for selecting drive routes for testing RF coverage in a radiotelephone cell.
2. Description of the Related Art
Cellular telephone systems are based upon two primary subsystems: base stations and mobile units. Each service provider is allocated a coverage area which is divided into a series of cells (typically 7), each further divided into a number of sectors (typically 3). In turn, each cell is controlled by a base station which communicates with up to a given maximum number of mobile units operating in the cell. Communications (transmission and reception) between the base station and the mobile units can be established in either an analog mode, such as AMPS (advanced mobile phone system,) or a digital mode, such as TDMA (time division multiple access) or CDMA (code division multiple access), as selected for a given system.
A key consideration in the design and operation of a cellular telephone system is the reliability of the radio frequency (RF) links between a given base station and the mobile units operating within the associated cell. Reliability is generally a predictor of reception quality as well as the probability that calls will be dropped. Two commonly applied methods of determining RF coverage reliability are: (1) cell edge reliability and (2) cell area reliability. Cell edge reliability refers to the probability that the RF signal strength measured on a contour at the cell edge will exceed a desired quality (signal strength) threshold (e.g., -90 dBm). Cell area reliability refers to the probability that the RF signal strength will meet or exceed the quality threshold after integrating the contour probability over the entire area of the cell. Often a service provider or system user will define minimums for both cell edge reliability and cell area reliability and, depending on the criticality of the given system, these reliability values may range from 70% to 99%. For example, for a system in which reliability is critical, the required contour reliability may be set at 95%, which generally translates to an area failure probability of %5.
Extensive RF field measurements are required in order to obtain the data necessary to calculate the requisite probabilities. Typically, field personnel travel from measurement point to measurement point taking signal strength and precise location measurements. For example, the received signal strength of an RF signal may be measured at a predetermined number of test points in relation to a central transmitter using a Received Signal Strength Indicator (RSSI). The exact location of each of these test points is determined by a GPS (global positioning system) receiver associated with the RSSI receiver. From the empirical signal strength and position data, the probability of the signal strength exceeding the set quality threshold at points along the cell edge and across the cell area can be determined.
The taking of signal strength samples requires substantial effort. Specifically, in order to establish acceptable probabilities, these measurements must be taken at hundreds of thousands of test locations. Further, the test locations cannot be too close or too far from the central receiver in order to properly model the coverage of the entire cell. Since the field personnel normally have to drive from location to location, the process of obtaining the proper number of samples at the proper locations can be time consuming and expensive.
Thus, the need has arisen for systems and methods which provide for the estimation of RF coverage within a cellular telephone system cell. Among other things, such systems and methods should provide for the streamlining of the process of obtaining test data and thus reducing the expense, time and number of personnel required to properly complete the task.